That's not substantiating your initial goalpost. You suggested Amazon wouldn't make Roombas better simply because they can preferentially sell their products. First, empirically, that is false given all other hardware devices Amazon makes have improved over time and not particularly gotten more expensive.

Second, preferentially selling it on one retailer does not suddenly make all the competition go away. Amazon is not a retail monopoly or anywhere close to it despite Lina Khan's bullshit paper that went through some mental gymnastics to create that myth. If anything, the synergy with the retail storefront can reduce overhead and decrease prices to the consumer.